Barred owls began their westward migration in the early 1900’s. Global warming may be one of the culprits behind the move – warming of just a few degrees might have allowed the owls to move into forests further to the north that connected East Coast and West Coast habitats. Human fire-suppression may also have opened up forest corridors that did not formerly exist. Photo: Dr. Lloyd Glenn Ingles, California Academy of Sciences

INVASIVE SPECIES: Barred Owls (Strix varia)

Most people associate the phrase “invasive species” with common pests, such as insects or weeds. However, birds and mammals can also wreak havoc on delicately-balanced habitats when they roam outside their natural range. The magnificent Barred Owl may be doing just this in the old-growth redwood forests of Oregon, Washington and California. Native to forests in the eastern United States and Canada, the Barred Owl first began appearing in northern West Coast forests in 1960. Today, these birds are common inhabitants of coastal forests as far south as Yosemite, and as their numbers grow, the owls that are native to the area – endangered Spotted Owls – are suffering population declines.

California Academy of Sciences ornithologist Jack Dumbacher is currently working to document the movement of Barred Owls into new territories and shed some light on the interactions between these two birds. Although he has only begun to collect data, early findings suggest that the Barred Owl may be at least partly responsible for the Spotted Owl’s decline. The two species appear to have very similar preferences for food and habitat conditions, and since the Barred Owl is slightly larger and more aggressive than the Spotted Owl, it may be a more successful competitor. There is even some evidence that Barred Owls have killed and eaten Spotted Owls. A less aggressive interaction – but one that could be equally damaging – is that the two closely-related species could interbreed, further diminishing the pool of purebred Spotted Owls.

By studying these interactions, Dumbacher hopes to provide data that will help forest managers make informed decisions about how best to protect both species of birds.